Nonaqueous electrolyte secondary battery

ABSTRACT

In a nonaqueous electrolyte secondary battery having a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a nonaqueous electrolyte, as the positive electrode active material or as the negative electrode active material, a mixture containing molybdenum dioxide and lithium titanate in a weight ratio (molybdenum dioxide:lithium titanate) of 90:10 to 50:50 is used.

This application is a division of application Ser. No. 11/656,008, filedJan. 22, 2007, which claims priority based on Japanese PatentApplication Nos. 2006-014988 and 2006-354764, filed Jan. 24, 2006, andDec. 28, 2006, respectively, and which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nonaqueous electrolyte secondarybatteries, and more specifically to a nonaqueous electrolyte secondarybattery which can be used as a power source for memory backup of aportable device.

2. Description of the Related Art

In recent years, high electromotive force nonaqueous electrolytesecondary batteries using nonaqueous electrolyte have been widely usedas secondary batteries of high output and high energy density. Suchnonaqueous electrolyte secondary batteries are used as a power sourcefor memory backup of portable device, as well as a main power source ofportable device, and in recent years, increase in energy density isdemanded not only in a main power source of portable device but also ina power source for memory backup.

As a secondary battery for memory backup, for example, a battery inwhich lithium cobaltate (LiCoO₂) is used as a positive electrode activematerial and lithium titanate (Li₄Ti₅O₁₂) having spinel structure isused as a negative electrode active material has already been broughtinto practical use. However, the density and weight specific capacity oflithium titanate used as a negative electrode active material are 3.47g/mL and 175 mAh/g, respectively, so that there is a problem that energydensity per volume is low. In contrast, molybdenum dioxide reverselyreacts with lithium in a similar potential region to that of lithiumtitanate and has density and weight specific capacity of 6.44 g/mL and210 mAh/g, respectively, and has higher volume energy density thanlithium titanate. Use of molybdenum dioxide as an alternative to lithiumtitanate enables increases in energy density per volume of battery.

For example, Japanese Patent Laid-open No. 2000-243445 proposes to uselithium-containing manganese oxide as a positive electrode activematerial and molybdenum dioxide for negative electrode.

A secondary battery for memory backup is mounted as a battery to beincorporated into a device, and used without a protective circuit fromview points of implementation area and cost. Therefore, it is assumedthat over discharge condition may occur as the condition that electriccurrent is not supplied from the main power source lasts for a longtime, and hence it is demanded that capacity decrease is small even ifover discharge cycle is conducted.

As described above, molybdenum dioxide is superior in energy density pervolume to lithium titanate. However, examination made by the presentinventor revealed that in a nonaqueous electrolyte secondary battery inwhich lithium cobaltate is used as a positive electrode active materialand molybdenum dioxide is used as a negative electrode active material,rapid decrease in capacity occurs with lapse of over discharge cycle,and sufficient cycle characteristic is not obtained.

Further, when molybdenum dioxide is uses as a negative electrode activematerial, expansion and contraction at the time of occluding andreleasing of lithium are large, so that it is impossible to obtainsufficient cycle characteristic.

It is an object of the present invention to provide a nonaqueouselectrolyte secondary battery which is applicable as a power source formemory backup, and having large battery capacity and excellent overdischarge cycle characteristic.

SUMMARY OF THE INVENTION

The present invention provides a nonaqueous electrolyte secondarybattery which comprises a positive electrode containing a positiveelectrode active material, a negative electrode containing a negativeelectrode active material, and a nonaqueous electrolyte, wherein as thepositive electrode active material or the negative electrode activematerial, a mixture containing molybdenum dioxide and lithium titanatein a weight ratio (molybdenum dioxide lithium titanate) ranging from90:10 to 50:50 is used.

In an over discharge condition of a nonaqueous electrolyte secondarybattery having a negative electrode using molybdenum dioxide, lithiumconcentration in molybdenum dioxide is extremely low, and electrodepotential is high. Examination made by the present inventor demonstratedthat in such a condition, molybdenum dioxide is extremely instable in anelectrolyte and molybdenum elutes into the electrolyte (see referenceexperiments as described later). It can be supposed that this elutingmolybdenum deposits on surface of negative electrode, and inhibitsoccluding and releasing of lithium, to cause decrease in capacity withover discharge cycle.

According to the present invention, by mixing lithium titanate intomolybdenum dioxide, elution of molybdenum is suppressed even in thecondition that lithium concentration in molybdenum dioxide is low (seereference experiments as described later). Although details about thereason for the above are not apparent, about 3.1 V (vs. Li/Li⁺) ofpotential is exhibited by only molybdenum dioxide, while about 2.9 V(vs. Li/Li⁺) of potential which is lower by about 0.2 V is exhibitedwhen lithium titanate is mixed. Molybdenum dioxide seems to easily eluteat more electropositive potential. By mixing with lithium titanate,electrode is less likely to have electropositive potential even in overdischarge condition. This may suppress elution of molybdenum. It isexpected that decrease in capacity due to over discharge cycle issuppressed because elution of molybdenum into electrolyte is suppressedin this manner.

In the case where molybdenum dioxide is used for a positive electrode,as the capacity is increased by elevating charging voltage, lithiumconcentration in molybdenum dioxide which is a positive electrode activematerial is very low, and in such a condition, elution of molybdenuminto electrolyte occurs as described above. This exerts adverse affecton cycle characteristic. According to the present invention, by mixinglithium titanate into molybdenum dioxide, even when molybdenum dioxideis used for a positive electrode, it is possible to suppress elution ofmolybdenum and improve the cycle characteristic.

When molybdenum dioxide is used for a positive electrode, for example,lithium metal, lithium containing graphite, Li—Al alloy, Li—Si alloy orthe like is used for a negative electrode, and a battery havingoperating voltage ranging from about 2.0 to 1.0 V can be realized.

In a battery used for memory backup, an operating voltage of the sameband region with that of a driving voltage of semiconductor for whichbackup is to be conducted is requested. Molybdenum dioxide and lithiumtitanate as an active material exhibit similar band regions of operatingvoltage, and when it is used for a negative electrode in combinationwith lithium cobaltate or the like, a battery having an operatingvoltage of about 3.0 to 2.0 V can be realized, whereas when it is usedfor a positive electrode in combination with carbon or aluminum, siliconor the like, a battery having an operating voltage of about 2.0 to 1.0 Vcan be realized. Therefore, by using molybdenum dioxide and lithiumtitanate as an active material, various requests for voltage bandregions can be satisfied.

Current largest market of secondary battery for backup use is forsecondary batteries which are chargeable and dischargeable in regionsfrom 3.0 to 2.0 V. As a positive electrode active material exhibitingcharge-discharge potential satisfying the above requirement, lithiumcobaltate is most preferably used. In the case of lithium nickelate,charge-discharge potential decreases and also discharge voltage ofbattery decreases, so that sufficient capacity is not obtained indischarge of up to 2.0 V. In the case of lithium manganate, a problemmay occur in storage characteristic.

Since lithium titanate and molybdenum dioxide have similar operatingpotentials at the time of occluding/releasing of lithium, and molybdenumdioxide may have higher electrode density compared to lithium titanate,it is possible to increase the energy density while keeping the voltagecompatibility with a conventional battery based on lithium titanate, byapplying mixture of molybdenum dioxide and lithium titanate as an activematerial according to the present invention.

As described above, an electrode using only molybdenum dioxide faces aproblem of poor cycle characteristic due to volume change or the like ofmolybdenum dioxide caused by charge and discharge. That is, electrolytethat is no longer retained in electrode due to expansion of molybdenumdioxide at the time of charging migrates to redundant space in thebattery system. However, at the time of discharging, contraction ofmolybdenum dioxide causes reabsorption of liquid, so that liquid willnot migrate smoothly from the redundant space. For this reason, in theelectrode using molybdenum dioxide exhibiting large volume change as anactive material, liquid retaining volume is reduced and charge-dischargecapacity is reduced as a result of repeated charges and discharges. Suchphenomenon is considered as a secondary factor of deterioration in cyclecharacteristic, in addition to elution of molybdenum in theaforementioned over discharge cycle.

Usually, electrolyte in an electrode is retained in carbon added as aconductive agent or in gap of active material or in gap betweenparticles. Since these volumes decrease with expansion of molybdenumdioxide, the liquid retaining ability will decrease. By adding inorganicporous particles such as alumina or titania in an electrode, volumes ofthese inorganic oxide particles will not change by charging ordischarging, so that surface or fine pores of particles function asliquid retaining space in the electrode, and decrease in cycle capacityretention rate can be prevented. However, since most of these inorganicporous particles lack ability of occluding/releasing lithium in themolybdenum dioxide charge-discharge band region, energy densitydecreases as a result of addition of these inorganic porous particles tothe electrode.

In the present invention, lithium titanate used in mixture withmolybdenum dioxide exhibits an operating voltage which is similar tothat of molybdenum dioxide, causes little voltage change due to chargingand discharging, and has an ability of occluding/releasing lithium, sothat it may be used as an active material. Therefore, by using lithiumtitanate as inorganic porous particles in combination with molybdenumdioxide according to the present invention, it is possible to secureretaining ability of electrolyte while suppressing decrease in energydensity, and hence it is possible to increase the cycle characteristic.

In the present invention, molybdenum dioxide and lithium titanate aremixed in a weight ratio (molybdenum dioxide:lithium titanate) of 90:10to 50:50. By setting the mixing amount of lithium titanate at 10 wt. %or more, it is possible to sufficiently suppress elution of molybdenumat decreased lithium concentration. Even when the mixing amount oflithium titanate exceeds 50 wt. %, ability of suppressing elution ofmolybdenum is little observed, and decrease in energy density due toincrease in mixing amount of lithium titanate occurs. Therefore, themixing amount of lithium titanate is preferably 50 wt. % or less. Theweight ratio between molybdenum dioxide and lithium titanate is morepreferably in the range of 90:10 to 70:30, and still preferably in therange of 80:20 to 70:30.

Preferably, molybdenum dioxide is based on stoichiometric composition ofMoO₂. When molybdenum dioxide having higher oxidation number such asMoO_(2.25) enters, initial efficiency may decrease and cyclecharacteristic may deteriorate. Also lithium titanate is preferablybased on stoichiometric composition of Li₄Ti₅O₁₂.

According to the present invention, in the electrode using mixture ofmolybdenum dioxide and lithium titanate as an active material, it ispreferred to use as a conductive agent, graphitized vapor-grown carbonfiber having lattice constant C₀ in the range of 6.7 Å≦C₀≦6.8 Å, a ratioL_(a)/L_(c) of crystallite dimensions (L_(a) and L_(c)) in a basalsurface (surface a) and in a lamination direction (surface c) in therange of 4≦L_(a)/L_(c)≦6. By using such graphitized vapor-grown carbonfiber as a conductive agent, it is possible to suppress decomposition ofelectrolyte on the conductive agent, and to realize a nonaqueouselectrolyte secondary battery having excellent cycle characteristic andstorage characteristic.

In theory, lower limit value of C₀ of graphite material is 6.7 Å. Thevalue of C₀ is preferably 6.8 Å or less because larger interlayerdistance may possibly accelerate a decomposition reaction of theelectrolyte. Since it is expected that most of side reaction such asdecomposition of electrolyte in graphite material occurs in surface c,and side reaction occurring in surface a is insignificant, it ispreferred to make exposure of surface c small. Therefore, the value ofL_(a)/L_(c) is preferably 4 or more. However, when L_(a) is too large,aspect ratio of fiber shape is large, and formability of electrode andhandling ability of combined material reduce, so that the value ofL_(a)/L_(c) is preferably 6 or less.

In the present invention, it is preferred to use as a conductive agent,massive artificial graphite having lattice constant C₀ in the range of6.7 Å≦C₀≦6.8 Å combined and mixed with the above vapor-grown carbonfiber. By combinational use of such massive artificial graphite as aconductive agent, it is possible to realize an electrode having highstrength and excellent productivity and high availability of activematerial. Mixing ratio of the vapor-grown carbon fiber and massiveartificial graphite is preferably in the range of 50:50 to 100:0 byweight ratio (vapor-grown carbon fiber:massive artificial graphite).Larger proportion of massive artificial graphite may possiblydeteriorate the cycle characteristic.

In the present invention, when mixture of molybdenum dioxide and lithiumtitanate is used as a positive electrode active material, for example, acarbon material such as graphite, metal which is able to form alloy withlithium such as aluminum, silicon or the like may be used as a negativeelectrode active material. By using these materials as a positiveelectrode active material, it is possible to realize a nonaqueouselectrolyte secondary battery having operating voltage of about 2.0 to1.0 V.

In the present invention, when mixture of molybdenum dioxide and lithiumtitanate is used as a negative electrode active material, lithiumcontaining transition metal complex oxide such as lithium cobaltatewhich is conventionally used as a positive electrode active material ofnonaqueous electrolyte secondary battery may be used as a positiveelectrode active material.

When lithium cobaltate is used as a positive electrode active materialand the above mixture is used as a negative electrode active material,use depth of lithium cobaltate is preferably in the range of 4.0 to 4.3V (vs. Li/Li⁺) in order to secure sufficient cycle characteristic. Inthe region of less than 4.0 V (vs. Li/Li⁺), sufficient specific capacityis not obtained, and in the region of more than 4.3 V (vs. Li/Li⁺),structure of active material is instable, and sufficient cyclecharacteristic may not be obtained. At charge-discharge depth of 4.0 V(vs. Li/Li⁺), specific capacity of lithium cobaltate is about 100 mAh/g,and at charge-discharge depth of 4.3 V (vs. Li/Li⁺), specific capacityof lithium cobaltate is about 165 mAh/g. Further, specific capacities oflithium titanate and molybdenum dioxide are about 175 mAh/g and about210 mAh/g, respectively. From these facts, denoting weight of lithiumcobaltate which is a positive electrode active material by W_(LCO),weight of molybdenum dioxide used as a negative electrode activematerial by W_(MoO2), and weight of lithium titanate used as a negativeelectrode active material by W_(LTO), they are preferably used in theranges that satisfy 100≦(175×W_(LTO)+210×W_(MoO2))/W_(LCO)≦165. Bysatisfying this condition, more preferred cycle characteristic isobtained.

In the present invention, as a nonaqueous electrolyte solvent, a solventthat contains 5-30% by volume of ethylene carbonate in the solvent ispreferred. With ethylene carbonate of less than 5% by volume, sufficientlithium ion conductivity may not be obtained in nonaqueous electrolyte.With ethylene carbonate of more than 30% by volume, a film isexcessively formed by decomposed matters of ethylene carbonate, withrespect to negative electrode active material, so that cyclecharacteristic may be deteriorated. As other solvent in nonaqueouselectrolyte, cyclic carbonate solvents such as propylene carbonate,butylene carbonate and the like, and chain-like carbonate solvents suchas diethyl carbonate, ethylmethyl carbonate, dimethyl carbonate and thelike can be used, and preferably, mixture of cyclic carbonate solventand chain-like carbonate solvent is desirably used.

As a solute of nonaqueous electrolyte in the present invention, lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), LiTFSI(LiN(CF₃SO₂)₂), LiBETI (LiN(C₂F₅SO₂)₂) and the like can be used.

According to the present invention, it is possible to provide anonaqueous electrolyte secondary battery having large battery capacityand excellent over discharge cycle characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic section view showing a lithium secondary batteryfabricated in Example according to the present invention; and

FIG. 2 is a graph showing relationship between mixing ratio of activematerial and capacity decrease rate by cycles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Example 1 Fabricationof Positive Electrode

LiCoO₂, acetylene black, artificial graphite, and polyvinylidenefluoride (PVdF) were mixed in weight ratio of 88.8:5:5:1.2 inN-methylpyrrolidone (NMP) solvent, and ground after drying to give apositive electrode combined material.

25.5 mg of the obtained positive electrode combined material wasweighed, and input into a molding jig having a diameter of 4.16 mm, andmolded under pressure of 600 kg·f to prepare a disc-shaped positiveelectrode.

[Fabrication of Negative Electrode]

After mixing as an active material, MoO₂ and Li₄Ti₅O₁₂ in a weight ratioof 90:10, the active material, graphitized vapor-grown carbon fiber(C₀=6.80 Å, L_(a)=900 Å, L_(c)=200 Å), massive artificial graphite(C₀=6.72 Å, L_(a)=300 Å, L_(c)=300 Å), and polyvinylidene fluoride(PVdF) serving as a binder were mixed in a weight ratio of 90:4:3:3, andground after drying to give a negative electrode combined material.

15.8 mg of the obtained negative electrode combined material wasweighed, and input into a molding jig having a diameter of 4.16 mm, andmolded under pressure of 600 kg·f to prepare a disc-shaped negativeelectrode.

[Preparation of Electrolyte]

Lithium hexafluorophosphate (LiPF₆) which is a solute was dissolved in amixed solvent of 3:7 (by volume) of ethylene carbonate and diethylcarbonate, so that a concentration was 1 mol/L, to prepare a nonaqueouselectrolyte.

[Assembling of Battery]

Using the above positive electrode, negative electrode and nonaqueouselectrolyte, a flat lithium secondary battery (battery dimension:diameter 6 mm, thickness 1.4 mm) was prepared. FIG. 1 is a schematicsection view showing a prepared lithium secondary battery. As shown inFIG. 1, a positive electrode 1 and a negative electrode 2 are arrangedto oppose each other via a separator 3, and housed in a battery casemade up of a positive electrode can 4 and a negative electrode can 5.The positive electrode 1 is connected to the positive electrode can 4and the negative electrode 2 is connected to the negative electrode can5, respectively via a conductive paste 7 made of carbon. Outer peripheryof the negative electrode can 5 is fitted inside the positive electrodecan 4 via a gasket 6 of polypropylene. As the separator 3, nonwovenfabric made of polypropylene is used, and the positive electrode 1,negative electrode 2 and separator 3 are immersed with the abovenonaqueous electrolyte.

Example 2

A lithium secondary battery was prepared in a similar manner to Example1 using a positive electrode and nonaqueous electrolyte similar to thoseof Example 1 except that MoO₂ and Li₄Ti₅O₁₂ serving as a negativeelectrode active material were mixed in a weight ratio of 75:25. Theamount of positive electrode combined material was 24.5 mg and theamount of negative electrode combined material was 15.5 mg.

Example 3

A lithium secondary battery was prepared in a similar manner to Example1 using a positive electrode and nonaqueous electrolyte similar to thoseof Example 1 except that MoO₂ and Li₄Ti₅O₁₂ serving as a negativeelectrode active material were mixed in a weight ratio of 50:50. Theamount of positive electrode combined material was 23.4 mg and theamount of negative electrode combined material was 15.4 mg.

Comparative Example 1

A lithium secondary battery was prepared in a similar manner to Example1 using a positive electrode and nonaqueous electrolyte similar to thoseof Example 1 except that only MoO₂ was used as a negative electrodeactive material. The amount of positive electrode combined material was26.4 mg and the amount of negative electrode combined material was 16.1mg.

Comparative Example 2

A lithium secondary battery was prepared in a similar manner to Example1 using a positive electrode and nonaqueous electrolyte similar to thoseof Example 1 except that only Li₄Ti₅O₁₂ was used as a negative electrodeactive material. The amount of positive electrode combined material was20.3 mg and the amount of negative electrode combined material was 14.4mg.

Constructions of batteries according to these Examples and ComparativeExamples are shown in Table 1.

TABLE 1 Mixing Ratio of Positive Negative Negative Electrode PositivePositive Electrode Negative Negative Electrode Active Material ElectrodeElectrode Filling Electrode Electrode Filling (175 × MoO₂ Li₄Ti₅O₁₂Weight Thickness Density Weight Thickness Density W_(LTO) + 210 × wt. %wt. % mg mm g/ml mg mm g/ml W_(MoO2))/W_(LCO) Comp. 100 0 26.4 0.59 3.316.1 0.311 3.83 129.8 Ex. 1 Ex. 1 90 10 25.5 0.568 3.3 15.8 0.333 3.49129.7 Ex. 2 75 25 24.5 0.547 3.3 15.5 0.353 3.24 129.0 Ex. 3 50 50 23.40.521 3.3 15.4 0.379 3.00 128.4 Comp. 0 100 20.3 0.516 3.3 14.4 0.3842.92 125.8 Ex. 2

[Evaluation of Charge-Discharge Characteristic]

Initial charge-discharge characteristic and charge-discharge cyclecharacteristic were evaluated for each of the above Examples andComparative Examples. Measurement conditions are as follows.

<Measurement Condition of Initial Charge-Discharge Characteristic>

Charge: constant current-constant voltage charge 100 μA-3.2 V 5 μA cut

Discharge: Step variable constant current discharge 100 μA, 50 μA, 30μA, 10 μA, 5 μA-2.0 V cut

Pause: 10 seconds

Initial charge capacity, initial discharge capacity and initialefficiency of each battery measured in the above measurement conditionsare shown in Table 2.

<Measurement Condition of Normal Cycle Characteristic>

Charge: constant current charge 100 μA 3.2 V cut

Discharge: constant current discharge 100 μA 2.0 V cut

Pause: 10 seconds

<Measurement Condition of Over Discharge Cycle Characteristic>

Charge: constant current charge 100 μA 3.2 V cut

Discharge: constant current discharge 100 μA 0.01 V cut

Pause: 10 seconds

Respective discharge capacities after 50 cycles measured in the aboveconditions are shown in Table 2 as discharge capacity after normal cycleand discharge capacity after over discharge cycle.

1-10 cyc. capacity decrease rate and 11-50 cyc. capacity decrease ratemeasured in accordance with the above measurement condition of overdischarge cycle characteristic are shown in FIG. 2. These capacitydecrease rates are calculated according to the following formula.

(1-10 cyc. capacity decrease rate)=100−(10 cyc. discharge capacity)/(1cyc. discharge capacity)×100(%)

(11-50 cyc. capacity decrease rate)=100−(50 cyc. discharge capacity)/(11cyc. discharge capacity)×100(%)

TABLE 2 Mixing Ratio of Negative Electrode Initial Initial DischargeCapacity Discharge Capacity Active Material Charge Discharge InitialAfter After Over MoO₂ Li₄Ti₅O₁₂ Capacity Capacity Efficiency NormalCycle Discharge Cycle wt. % wt. % mAh mAh % mAh mAh Comp. 100 0 3.192.81 87.9 1.71 0.28 Ex. 1 Ex. 1 90 10 3.12 2.74 88.0 1.75 0.96 Ex. 2 7525 2.99 2.64 88.4 1.72 1.63 Ex. 3 50 50 2.71 2.42 89.2 1.75 1.86 Comp. 0100 2.38 2.19 92.0 1.66 1.38 Ex. 2

As is apparent from the result shown in Table 2, in Examples 1 to 3using as a negative electrode active material, a mixture containing 10to 50% by weight of lithium titanate, relative to the total amount ofmolybdenum dioxide and lithium titanate according to the presentinvention, high initial charge capacity and initial discharge capacity,excellent initial efficiency, and high discharge capacity after 50cycles in over discharge cycle are observed, demonstrating excellentover discharge cycle characteristic. As to normal cycle characteristic,such a significant difference as is the case of over discharge cycle wasnot observed between Examples 1 to 3 according to the present inventionand Comparative Examples 1 and 2. This reveals that the effect of thepresent invention is particularly outstanding in over discharge cyclecharacteristic.

As is apparent from FIG. 2, decrease in capacity is notably suppressedin the cycles following cycle 11.

<Reference Experiments>

(Reference Experiment A)

A slurry prepared by mixing molybdenum dioxide, graphitized vapor-growncarbon fiber and PVdF in a weight ratio of 90:5:5 in NMP solvent wasapplied and dried on aluminum foil and then compressed to form anapplied polar plate. This was then cut into a rectangular shape of2.5×5.0 cm. The amount of molybdenum dioxide in the polar plate was147.3 mg. This polar plate was immersed with a nonaqueous electrolyte(1M (mol/litter) LiPF₆ EC/DEC=3/7), and then stored for 5 days at 60° C.while the nonaqueous atmosphere was kept, and Mo element eluted into theelectrolyte was quantified by using ICP. The proportion of quantity ofeluted Mo element, relative to quantity of Mo element contained in thepolar plate before storage was 350.7 ppm.

(Reference Experiment B)

After electrically inserting lithium into a polar plate which isidentical to that used in Reference Experiment A until Li_(0.25)MoO₂(1.6 V(vs.Li/Li⁺)) was achieved, storage was conducted in the samemanner as in Reference Experiment A, and Mo element eluted into theelectrolyte was quantified by using ICP. The proportion of quantity ofeluted Mo element, relative to quantity of Mo element contained in thepolar plate before storage was 93.6 ppm.

(Reference Experiment C)

A slurry prepared by mixing mixture of 75:25 (by weight) of molybdenumdioxide and lithium titanate, vapor-grown carbon fiber and PVdF in aweight ratio of 90:5:5 in NMP solvent was applied and dried on aluminumfoil and then compressed to form an applied polar plate. This was thencut into a rectangular shape of 2.5×5.0 cm. The amount of molybdenumdioxide in the polar plate was 110.7 mg. Storage in electrolyte wasconducted in the same manner as in Reference Experiment A, and Moelement eluted into the electrolyte was quantified by using ICP. Theproportion of quantity of eluted Mo element, relative to quantity of Moelement contained in the polar plate before storage was 80.2 ppm.

From comparison between Reference Experiment A and Reference ExperimentB, it was found that molybdenum dioxide is easy to elute especially whenthe lithium concentration in polar plate is low.

From comparison between Reference Experiment A and Reference ExperimentC, it was found that by mixing lithium titanate into molybdenum dioxide,elution of molybdenum is suppressed even when lithium concentration inmolybdenum dioxide is low.

1. A nonaqueous electrolyte secondary battery comprising: a positiveelectrode containing a positive electrode active material; a negativeelectrode containing a negative electrode active material; and anonaqueous electrolyte, wherein as the positive electrode activematerial or as the negative electrode active material, a mixturecontaining molybdenum dioxide and lithium titanate in a weight ratio(molybdenum dioxide lithium titanate) of 90:10 to 50:50 is used.
 2. Thenonaqueous electrolyte secondary battery according to claim 1, whereinin the positive electrode or negative electrode using the mixture, as aconductive agent, graphitized vapor-grown carbon fiber having latticeconstant C₀ in the range of 6.7 Å≦C₀≦6.8 Å, a ratio L_(a)/L_(c) ofcrystallite dimensions (L_(a) and L_(c)) in a base surface (surface a)and in a lamination direction (surface c) in the range of4≦L_(a)/L_(c)≦6 is used.
 3. The nonaqueous electrolyte secondary batteryaccording to claim 2, wherein as the conductive agent, massiveartificial graphite having lattice constant C₀ in the range of 6.7Å≦C₀≦6.8 Å is used in mixture with the vapor-grown carbon fiber.
 4. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe mixture of molybdenum dioxide and lithium titanate is used as thenegative electrode active material, and lithium cobaltate is used as thepositive electrode active material.
 5. The nonaqueous electrolytesecondary battery according to claim 4, wherein when weight of lithiumcobaltate used as the positive electrode active material is denoted byW_(LCO), weight of molybdenum dioxide used as the negative electrodeactive material by W_(MoO2), and weight of lithium titanate used as thenegative electrode active material by W_(LTO),100≦(175×W_(LTO)+210×W_(MoO2))/W_(LCO)≦165 is satisfied.
 6. Thenonaqueous electrolyte secondary battery according to claim 1, whereinas a solvent of the nonaqueous electrolyte, 5-30% by volume of ethylenecarbonate is contained in the solvent.
 7. The nonaqueous electrolytesecondary battery according to claim 2, wherein as a solvent of thenonaqueous electrolyte, 5-30% by volume of ethylene carbonate iscontained in the solvent.
 8. The nonaqueous electrolyte secondarybattery according to claim 3, wherein as a solvent of the nonaqueouselectrolyte, 5-30% by volume of ethylene carbonate is contained in thesolvent.
 9. The nonaqueous electrolyte secondary battery according toclaim 4, wherein as a solvent of the nonaqueous electrolyte, 5-30% byvolume of ethylene carbonate is contained in the solvent.
 10. Thenonaqueous electrolyte secondary battery according to claim 2, whereinthe mixture of molybdenum dioxide and lithium titanate is used as thenegative electrode active material, and lithium cobaltate is used as thepositive electrode active material.
 11. The nonaqueous electrolytesecondary battery according to claim 10, wherein as a solvent of thenonaqueous electrolyte, 5-30% by volume of ethylene carbonate iscontained in the solvent.
 12. The nonaqueous electrolyte secondarybattery according to claim 3, wherein the mixture of molybdenum dioxideand lithium titanate is used as the negative electrode active material,and lithium cobaltate is used as the positive electrode active material.13. The nonaqueous electrolyte secondary battery according to claim 12,wherein as a solvent of the nonaqueous electrolyte, 5-30% by volume ofethylene carbonate is contained in the solvent.
 14. The nonaqueouselectrolyte secondary battery according to claim 10, wherein when weightof lithium cobaltate used as the positive electrode active material isdenoted by W_(LCO), weight of molybdenum dioxide used as the negativeelectrode active material by W_(MoO2), and weight of lithium titanateused as the negative electrode active material by W_(LTO),100≦(175×W_(LTO)+²¹⁰×W_(MoO2))/W_(LCO)≦¹⁶⁵ is satisfied.
 15. Thenonaqueous electrolyte secondary battery according to claim 12, whereinwhen weight of lithium cobaltate used as the positive electrode activematerial is denoted by W_(LCO), weight of molybdenum dioxide used as thenegative electrode active material by W_(MoO2), and weight of lithiumtitanate used as the negative electrode active material by W_(LTO),100≦(175×W_(LTO)+210×W_(MoO2))/W_(LCO)≦165 is satisfied.